Recently, hydrofluorocarbon(s) (HFC), which has zero ozone-depletion potential, has been used as a refrigerant for a refrigerating machine and the like. However, HFC has high global-warming potential (GWP), and hence has been required to be replaced with a refrigerant with low GWP. A fluoropropene refrigerant, HFO-1234ze, has been considered as one of the candidates (Patent Literature 1).
A refrigerant-circulation cycle of a refrigerating machine or the like generally has a structure in which a refrigerating machine oil for lubricating a refrigerant compressor is circulated together with a refrigerant in the cycle. For this reason, a refrigerating machine oil is required to have miscibility with the refrigerant (refrigerant miscibility). In addition, since the refrigerating machine oil is used to lubricate the operational parts of a refrigerating machine, the lubricity is, of course, important. When phase separation of the refrigerant and the refrigerating machine oil occurs, the refrigerating machine oil discharged from the refrigerant compressor tends to build up in the cycle. As a result, problems such as a lubrication failure due to decrease in amount of the refrigerating machine oil in the refrigerant compressor and clogging of an expansion mechanism such as a capillary may occur (Patent Literatures 2 and 3).
On the other hand, when a refrigerating machine oil has a good miscibility with a refrigerant, the refrigerant is dissolved in the refrigerating machine oil in a refrigerant-circulation cycle, so that the viscosity (refrigerant solution viscosity) of a fluid composition which is a mixture of the refrigerating machine oil and the refrigerant decreases, making it difficult to retain the oil film necessary for the lubrication. For this reason, a problem of lubrication failure may arise. Especially, since a refrigerant consisting of HFO-1234ze (hereinafter, referred to as “HFO-1234ze refrigerant”) has an extremely high miscibility with a refrigerating machine oil, the decrease in the refrigerant solution viscosity is remarkable. Note that it is possible to increase the viscosity of the refrigerating machine oil as a method for improving the lubricity. However, this is not preferable from the viewpoint of energy-saving (Patent Literature 4).
In addition, a refrigerating machine oil is required to have a high thermal and chemical stability enough to be used for a long period in the presence of a refrigerant. The HFO-1234ze refrigerant, which has an unsaturated bond, undergoes oxidative decomposition to generate an acid, and further the generated acid promotes hydrolysis of the refrigerating machine oil. For this reason, thermal and chemical stability of the refrigerating machine oil in a state where air or water is contained is especially important.
Each of Patent Literatures 1 and 5 discloses a refrigerating machine oil comprising an ester of a polyol and a fatty acid having 5 to 9 carbon atoms used in combination with a refrigerant containing HFO-1234ze. However, none of Patent Literatures 1 and 5 describe or suggest specific miscibility with the HFO-1234ze refrigerant, lubricity, or thermal and chemical stability.
Patent Literature 4 describes a refrigerating machine oil comprising an ester between pentaerythritol and a fatty acid B in which the ratio of branched fatty acids having 10 to 13 carbon atoms is 98.0% by mole. However, Patent Literature 4 does not disclose specific components of the branched fatty acids having 10 to 13 carbon atoms constituting the ester. In addition, Patent Literature 4 neither describes nor suggests specific miscibility with the HFO-1234ze refrigerant, thermal and chemical stability, lubricity, or the like.
Patent Literature 6 describes a tetraester between 2-propylheptanoic acid and pentaerythritol as a lubricant base stock for an automotive, an aeronautic or the like engine or turbine. However, Patent Literature 6 neither describes nor suggests the miscibility with refrigerant, the lubricity, or the thermal and chemical stability of the ester in the presence of the HFO-1234ze refrigerant.